The present invention generally relates to vehicle brake systems, and more particularly relates to a method for controlling a brake system for automotive vehicles including a vehicle speed control unit permitting the adjustment of a predetermined vehicle speed by way of an automatic intervention into brake control.
Brake control systems including a vehicle speed control unit which adjusts a predetermined vehicle speed by way of an automatic brake intervention are known in the art. Depression of the brake pedal by the driver causes the vehicle speed control unit to change from the activated into the deactivated mode, with the result that an automatic brake intervention operation performed by the vehicle speed control unit is terminated. The pressure which recently prevailed in the wheel brakes due to the automatic brake intervention is reduced until the brake pressure introduced into the wheel brakes rises again due to application of the brake pedal. This causes a jerk (discontinuity) in the vehicle deceleration.
EP 0 867 349 discloses a method intended to prevent this break in the vehicle deceleration. It is proposed therein that after the deactivation of the vehicle speed control unit at the beginning of the application of the brake pedal, the actual pressure which was adjusted the last by the vehicle speed control unit is maintained or continuously adapted to a higher nominal pressure demanded by the driver, or a total pressure is adjusted which corresponds to the sum of the actual pressure and the nominal pressure. The brake pressure predetermined by the vehicle speed control unit is not reduced when the brake pedal is applied, but rather maintained or increased, with the aim of providing jerkfree transitions from an activated mode into a deactivated mode of the vehicle speed control unit.
In a brake system with an active, electrically actuated brake force booster, the brake pedal is moved along with an automatic brake intervention of the vehicle speed control unit. When the driver wishes to brake more vigorously or deactivate the vehicle speed control unit, he/she will apply the brake pedal at that point and with the force that corresponds to the brake pressure in the wheel brake. In a so-called xe2x80x98brake-by-wire brake systemxe2x80x99 where the transmission of the driver""s braking request from the brake pedal to the hydraulic actuators, when an electrohydraulic brake (EHB) is concerned, or to the electromechanic actuators, when an electromechanic brake (EMB) is concerned, is effected electrically, the brake pedal remains in the inactive position. The take-over by the driver during a braking operation initiated by the vehicle speed control unit is not transparent for the driver, especially in a brake-by-wire brake system.
An object of the present invention is to improve the method according to the state of the art and, especially for a brake-by-wire brake system, to render the take-over of the control of the braking operation from an automatic brake intervention of the vehicle speed control unit to a brake actuation determined by the driver comfortable and also transparent.
The essence of the present invention resides in that after the function of the vehicle speed control unit is deactivated in the event of a braking request of the vehicle speed control unit and, simultaneously, a braking request of the driver, the pressure (PA) introduced into the wheel brake due to the braking request of the vehicle speed control unit is reduced according to a predetermined function (PX) upon the braking request of the driver.
The reduction of the pressure (PA) introduced into the wheel brake in response to the braking request of the vehicle speed control unit permits the driver to recognize when the application of the brake pedal leads to a brake intervention controlled by the driver. The reduction is performed according to a predetermined function which is so chosen that the brake application is comfortable and also transparent for the driver and, in addition, a maximum high deceleration is ensured in an emergency situation (panic stop).
According to the present invention, an at least approximately linear ramp function is predetermined as the function.
The predetermined function has a mean (negative) gradient (mPX) of (20 bar/5 sec) to (30 bar/0.5 sec), preferably of (20 bar/2 sec) to (30 bar/1 sec) as disclosed by the present invention.
In the present invention, the function is changed in accordance with the braking request of the driver.
According to the present invention, a function is predetermined upon a request of the driver for quick braking of the vehicle causing quick reduction of the pressure (PA) introduced into the wheel brake due to the braking request of the vehicle speed control unit, and upon a request of the driver for slow braking of the vehicle a function is predetermined causing slow reduction of the pressure (PA) introduced into the wheel brake due to the braking request of the vehicle speed control unit.
It is arranged for by the present invention that a mean (positive) gradient (mPF) of the brake pressure is determined from the braking request of the driver, and therefrom the mean (negative) gradient (mPX) of the predetermined function is defined by the formula mPX=xe2x88x92mPF*constant K, until a top limit value of 100 bar/sec). This means a maximum mean (negative) gradient (mPX) and, thus, a maximum pressure reduction speed of (100 bar/sec) is predetermined. The constant K has preferably a value in the range from 1 to 2, particularly preferred is a value of 1.5.
Because it is desired that brake pressure reduction is carried out in each case relatively quickly after deactivation of the vehicle speed control unit, it is arranged for that the value for the mean (negative) gradient (mPX) of the predetermined function does not fall below a minimum limit value (lowest pressure reduction speed) in a range of (20 bar/5 sec) until (20 bar/2 sec), preferably 20 bar/3 sec).
It is provided according to the present invention that a resulting brake pressure (Pres) in the wheel brake of the vehicle is adjusted in accordance with the pressure (PF) introduced into the wheel brake due to the braking request of the driver, and the pressure (PX) introduced into the wheel brake due to the predetermined function.
According to an embodiment of the present invention, a resulting brake pressure (Pres) is adjusted in the wheel brake of the vehicle which corresponds to the sum of a pressure (PF) introduced into the wheel brake due to the braking request of the driver and the pressure (PX) introduced by the predetermined function into the wheel brake.
According to another embodiment of the present invention, a resulting brake pressure (Pres) is adjusted in the wheel brake of the vehicle which corresponds to the higher of the two pressures, i.e., the pressure (PF) introduced into the wheel brake due to the braking request of the driver or the pressure (PX) introduced by the predetermined function into the wheel brake.
In another embodiment of the present invention, a resulting brake pressure (Pres) is adjusted in the wheel brake of the vehicle which corresponds to a value that is lower than the sum of the pressure (PF) introduced into the wheel brake due to the braking request of the driver and the pressure (PX) introduced by the predetermined function into the wheel brake, and which is higher than the higher one of the two pressures, i.e., the pressure (PF) introduced into the wheel brake due to the braking request of the driver or the pressure (PX) introduced by the predetermined function into the wheel brake.
The latter embodiment of the present invention is especially preferred because it renders the brake pressure take-over by the driver especially comfortable and also transparent. According to the present invention, it has proved particularly favorable to adjust a resulting brake pressure (Pres) in the wheel brake of the vehicle which corresponds to a value that is determined by the formula Pres=PF+PXxe2x88x92(PF*PX)/constant Z.
According to the present invention, the constant Z (adjustment factor) has a value in the range of 20 bar to 100 bar, preferably 30 bar to 70 bar.
It is in the sense of the present invention that the brake pressure in the wheel brake is generated by independent energy, in particular, by a hydraulic accumulator and a motor-and-pump assembly. Preferably, the brake pressure request of the driver is sensed by means of a pedal travel simulator and associated sensors and transmitted electronically to a brake control unit (brake-by-wire brake system).
Further, the object is achieved by a device for controlling a brake system for automotive vehicles with a vehicle speed control unit which includes means to adjust a predetermined vehicle speed by means of an automatic intervention into the brake control and means to deactivate the function of the vehicle speed control unit in the event of a braking request of the vehicle speed control unit and, simultaneously, a braking request of the driver, the said device being characterized by the provision of means to reduce the pressure (PX) introduced into the wheel brake due to the braking request of the vehicle speed control according to a predetermined function unit in the event of the braking request of the driver, and by the provision of means to adjust in particular a resulting brake pressure (Pres) in the wheel brake of the vehicle that is determined by the formula Pres=PF+PXxe2x88x92(PF*PX)/constant Z.
Preferably, an independent-force-actuated, preferably electronically controllable vehicle brake, especially brake-by-wire vehicle brake, includes the device of the present invention for controlling the vehicle brake system.